1. Field of the Invention
The invention relates to a control device for a hybrid vehicle that is configured to include an engine, an electric motor, and a damper that is interposed in a motive power transmission path between the engine and the electric motor.
2. Description of Related Art
A hybrid vehicle that is configured to include an engine, an electric motor, and a damper that is equipped with a hysteresis mechanism in a motive power transmission path between the engine and the electric motor is well known. For example, a motive power transmission device described in Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A) is also one such example. In a damper of Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A), a large hysteresis torque is generated upon fluctuations of a twist angle in a negative-side twist range where a torque is transmitted from driving wheel sides. Thus, abrupt torque fluctuations that are caused in starting and stopping the engine are effectively damped. Besides, torque fluctuations during engine steady operation are effectively damped by generating a small hysteresis torque upon fluctuations in the twist angle in a positive-side twist range where a torque is transmitted from the engine side.
Besides, in a hybrid vehicle in general, the start and stop of an engine are frequently repeated. A plurality of control methods regarding the control of starting and stopping this engine have been proposed. For example, in a vehicular motive power transmission device described in Japanese Patent Application No. 2010-167921, a negative torque in such a direction as to reduce the rotational speed of an engine is output from an electric motor in stopping the engine, the torque is removed immediately before stop of the engine, and a positive torque in such a direction as to increase the rotational speed of the engine is slightly output to prevent reverse rotation of the engine.
By the way, it is known that if the negative torque of the electric motor that is output to reduce the rotational speed of the engine is removed immediately before the stop of rotation as in Japanese Patent Application No. 2010-167921 during stop control of the engine, the reactive force resulting from compression in a combustion chamber of the engine cannot be suppressed, the magnitude of torque fluctuations increases, and gear rattle noise is generated as a result of the torque fluctuations. Besides, the damper for the hybrid vehicle of Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A) has a characteristic that a small hysteresis torque is generated if the relative twist angle is equal to or smaller than a predetermined value in a twist in a positive direction in which a torque is transmitted from the engine side. In a damper having a twist characteristic (a hysteresis characteristic) as in this Japanese . Patent Application Publication No. 2006-29363 (JP-2006-29363 A), in the case where engine stop control as in Japanese Patent Application No. 2010-167921 is performed, if the removal of the torque is started from a state where the torque of the electric motor is a negative torque, the range where the small hysteresis torque is generated is utilized. Besides, even if a positive torque is output from the electric motor, the value thereof is small. Therefore, the damper may be twisted in the positive direction upon vibrations thereof, and the small hysteresis torque is generated at that time. Accordingly, the torque fluctuations that are caused in stopping the engine cannot be effectively damped by the large hysteresis torque, and it is difficult to suppress gear rattle noise resulting from the torque fluctuations.